Obtaining optical measurements (for example a light absorption, reflection or scattering measurement) of a liquid is a useful tool in many applications. In particular, optical measurements may be used to determine characteristics (absorbance, turbidity) of a sample or a component of a sample; or to monitor the kinetics of a reaction. In order for such measurements to be carried out, the liquid is placed in a receptacle and exposed to a light beam. Light which is transmitted through, the liquid is then analysed and characteristics of the liquid can be determined based on this analysis. The analytical procedure may involve single-point, multiple-point or time-resolved measurements.
Optical measurements are used in ‘point of care’ applications to analyse blood samples. A device is used to hold the sample relative to an optical unit (for producing a light beam and detecting transmitted or reflected light to infer, for example, the light absorbed or scattered by the liquid). Such devices may be microfluidic devices and/or may be centrifugal devices, such as ‘lab on a disc’ devices.
As well as obtaining an optical measurement of a liquid sample, such as a blood sample, it may be desirable to obtain a reference measurement also, for example a measurement of a buffer or saline solution, for comparison with the result obtained for the liquid sample. This can facilitate correction of results for imperfections in the receptacle in which the liquid is held, the material through which the light beam passes before and/or after passing through the liquid under measurement and/or imperfections in the optical unit, for example. It may also be advantageous to ascertain the optical properties of the reagents being used, in particular when these reagents are stored dry in the device and reconstituted at the moment of testing. Such reference measurements may enable the presence of substances in the sample which may interfere with the optical signal used for detection to be taken into account.